This invention relates to a starter-generator for an aircraft engine, such as a turbine engine.
A turbine engine employed in an aircraft may be started by supplying compressed air to an accessory air turbine motor having reduction gearing to drive the engine. Compressed air is provided by an auxiliary power unit. These pneumatic systems require numerous air ducts, seals and air valves that are not only bulky but heavy. Moreover, these systems add undesirable complexity to the aircraft, reducing reliability and increasing cost for aircraft operators.
Recently, aircraft manufacturers have commenced using electric starters for turbine engines. Such a starter adds little additional componentry and wiring to the aircraft because the starter takes advantage of the aircraft's existing electrical system. Thus, the starter does away with many of the components used to start the engine by compressed air.
One approach to starting a turbine engine electrically is to employ a single dynamoelectric machine that operates as both a starter and a generator. Typically, this machine comprises a rotor and stator that serve the dual function of cranking the engine to start and operating as a generator driven by the turbine engine after start. The machine supports this dual function to eliminate the need for separate machines, associated mounting pads, and gearing on the engine accessory gearbox. One such starter-generator system uses a synchronous generator to operate as an induction motor to start the turbine engine. However, the use of such a device as an induction motor to start the engine creates the risk of damaging the integrity of the device. Potential burning of rotating diodes, very high current through the damper bars and the effect of inrush currents on field windings all pose risks to the device.
One kind of dynamoelectric machine that may be employed as a starter-generator is a variable frequency generator. The generator is used with a variable frequency electrical system of an aircraft that has componentry receptive to electrical voltage at a frequency that may vary with engine speed. Such a system allows the starter-generator to be restarted by an auxiliary power unit or another variable frequency power source, such as from another generator driven by an engine in mid-flight.
The need to avoid overburdening of the variable frequency starter-generator during start-up is of particular importance because of the need to preserve the mid-flight restart capability of the aircraft. That is, a starter-generator burned out by overloading during start-up will not be useful in a restart condition.
A need therefore exists to safeguard the variable frequency dynamoelectric machine during engine start up.